A method of manually making thin sections is presented. Simple techniques for maintaining parallelism that avoids direct measurement is described in detail and a number of examples are shown.
Most amateur paleontologists never bother to look at their specimens with a magnifying glass let alone a microscope. They do not notice the detail available in most fossils and the abundance of microscopic flora and fauna that exists at this scale. These microfossils can be individual tiny fauna, juvenile live stages of larger fauna, or parts of larger fauna. Soft shales and friable limestones offer opportunities to free microfossils from matrix without resorting to drastic chemical methods, but too often the microfossil hunter comes across exquisite fossils in glass hard matrix. In such cases the difference in hardness between fossil and matrix is usually so close that there seems no way to free and exhibit the fossils. Thin sectioning these hard rocks is a means to not only enjoy these fossils, but in many of cases may be the only means of accurately classifying the fossils. This can lead the amateur paleontologist on an exploration in the microstructure of some of the largest megafossils. (Jones 1956)
The techniques described here are simple but can lead to remarkable results. This was gleaned from several sources and tried over the last few years. (Allman & Lawrence 1982, Kummel & Raup 1965) Patience is required for rushing the job will inevitably lead to ruin of the specimen and a waste of the time put in to the work. Materials, for the most resourceful amateur, can literally cost nothing and at most will cost little.
The task is to find a good specimen, grind and polish one face of a small cut out slab, glue that to a glass slide. Next, grind down and polish the other face of the specimen so that it is parallel to the first side and only about 0.03-0.05mm thick.
Finding a specimen.
Check the matrix around some of your fossils and you will likely find pill-shaped, torpedo-shaped and round inclusions. If you are lucky and the formation is rich, you may find identifiable fossils that are smaller (juvenile) versions of the megafossils. In the field you might find a rock that looks for all the world like a collection of spherical or cigar shaped grains. If you know the matrix to contain abundant megafossils then it is likely full of microfossils and you would do well to bring back a rock of the material. Many is the time I found a rock that only looked mildly promising and upon making the thin section found it to be abundantly rich in fauna. I have yet to find a promising rock that proved to be uninteresting.
Cutting the specimen out.
Back in the lab (read garage or basement!) cut off a piece of one end of the rock. The harder the rock is the better. You don't want it crumbling. (We will deal with the crumbly ones later.) You do not need to cut off more than a one inch square piece and would be better off if it were less for a first effort. If you do not have a motorized saw, a simple hack saw should do well. Cut another piece off the main rock as thin as you can, 2-4mm will do but 1-2mm is better. If it breaks then save the pieces. The important part is that you have a little slab with roughly flat, parallel sides under 1 square inch area.
Materials for grinding.
For the next steps a selection of grinding powders will be needed along with some other items:
More of the coarse powders will be needed and less of the fine which should please your wallet. These can be found at most any rock shop. There is no need to buy better grades and these are only recommendations. Substitutions can be made as to powder composition and grade. At minimum a coarse, medium and fine grade is required. Diamond polishing paste while not only expensive is too hard for this work. Titanium oxide is too slow and soft. Cerium offers a good compromise and is a very fast polisher especially if attention is paid to the last grinding step to make sure it is "fined" thoroughly.
You will also need some glass blanks. The glass should be at least 1/4" thick and about 4" round or square. If they are three inches that will work but the work will go a bit slower. If the blanks are square and you have a glass cutter, it is advisable to cut off the corners thus making an octagon. Be sure to grind off all sharp edges with a whetstone or a fine grinding stone on a grinder. This will save you a bloody mess should you slip when grinding. The glass will likely not be truly flat when you get or cut them. Check them by bouncing light off the surface at a very low angle look to see if any light passes under the edge of a ruler laid across it on edge (see diagram at end of paper). Make sure it is a good quality ruler (available at good hardware stores). Ideally, if the glass is dead flat, no light will be seen below the ruler especially after the glass is ground a bit. Anywhere a thin thread of light can be seen is a low spot. With a freshly cut piece of glass it is probable that the very edge will be higher than the middle. This is due to a problem called the Twyman Effect. There is a lot of tension stored in a polished glass surface and when it is cut the surface of the glass relaxes almost always resulting in a "turned-up-edge". This can be ground off easily using the instructions below.
Truing the glass blanks.
Glass blanks can be brought into flatness by grinding several against each other. This is an old technique used by opticians for several centuries. (Since I have been grinding telescope optics since 1965, this was "old-hat" to me.)
To start take two of the glass disks and put one on some newspaper on a good flat surface. I prefer working in an old baking pan or cafeteria tray to confine the slopped abrasive and run-off water. To begin a "wet" put a "charge" of the 220 grit on the bottom glass disk with a little water. Grind another similar sized disk on top back and forth with a stroke that causes and overlap of about a quarter of the disk diameter. Keep rotating the top piece every few strokes and the bottom piece, in the opposite direction, every minute or so. As you grind the top piece will become concave and the bottom convex. So if you started with a piece that was ground down in the center and thus concave and put that down as the bottom piece, eventually it will be flat again by this process. You will have to check with the ruler every few minutes. To make them initially flat should only take five to ten minutes.
If you are working only one specimen initially, have the turned-up-edge problem and the specimen is fairly thick piece of rock (in the 4mm range) you could just grind in a circular motion around the edge of the blank for the coarsest grade and check it afterwards. It will probably be pretty flat by then. Grinding. It is useful to define two terms here. Grinding is where a loose abrasive will be used between two unyielding surfaces, as in the case of rock and a piece of glass. It can also be where the abrasive is held by an unyielding medium like with diamond and carborundum grinders and grinding wheels. Polishing, on the other hand, is where the abrasive is held by a softer yielding medium and it scours the object pushed against it. In polishing there is also an micro-melting effect, but it is not necessary to go into that here. Take the little slab, place it on the glass tool with some abrasive and water from a rinse water bowl or pan that you should have nearby. Use only one finger on the back of the piece and grind with a medium to light pressure and move in a zig-zag or circular (or epicycular) motion around the glass tool (see diagram at end of paper). Be sure to move about the glass plate as best you can so you don't grind a low spot in one area unless you are trying to grind down a specific area of the tool (as in the example mentioned above). Grind it until all the saw marks are gone. Use a 10x handlens to inspect the piece as you grind it. If you had a steady hand on the saw and the cut marks have little relief, you could start with the 400/600 abrasive size. After each abrasive be sure to inspect the surface to see if the pits left by the abrasive are uniform and that all the bigger pits from the previous abrasive have been removed. Be sure to wash up the work completely between abrasive. One grain of a coarser grit in the finer grades will ruin your hard work. All this is particularly important if you are only using the three minimum grades of abrasive for grinding. I use the cafeteria tray with newspaper in the bottom and toss out the paper after every grade plus wash the tools, specimens, and change the rinse water. Once all the saw marks have been ground off the specimen and you are sure on inspection that the piece is still flat and the grind pattern on the plate is even, you can go on to the next abrasive. I go straight from 220 to 400 and then 5 micron. This is the minimum amount of steps you should use but you can omit the 220 if the original saw cut is smooth enough.
Again push the piece around in the abrasive with a medium to light pressure. Don't rush the grinding or you will break the specimen. If in the finest stage the piece kicks and sticks, try using a thicker slurry of abrasive and less pressure. If it still sticks you may want to wash it off and check the glass blank to see that it is still flat. If the rock is concave and the blank a flatter curve the work will kick and stick badly. It is unlikely that this will be a problem with a first piece done alone but may if you do a few at a time. In order to have a problem with this you would have to be a few hundredths of a millimeter out of flatness so it is advisable to only do a couple pieces at a time. If you do have to re-true the tool, be sure to do a minute or two of grinding with the previous abrasive to make the specimen match the new curve of the tool.
We now need that yielding medium to hold our cerium polishing compound. It also needs to be firm enough to not change the shape of the ground surface of the specimen. Go to your household rag-bag and get a piece of twill (from and old pair of workpants) or denim. The piece needs to be only half again longer than the diameter of the glass tool and half as wide. Get it wet and wrap it as tightly as possible around the glass tool. This is tricky but not impossible. The object is now to put some cerium polishing compound on the cloth and rub the ground surface over the cloth so it does not bunch or catch on the specimen. This may take some practice. You need not work it in one of the aforementioned patterns as we are now not wearing down the glass tool. Just rub it back and forth on the cerium charged cloth until you get a good polish. If you want to spend the money there are supply houses that carry self-adhesive polishing pads but the simple piece of cloth will work very well.
There are a number of different micro-slides available (medical, petrographic etc.) plus you can make your own like the ones I make to fit a 35mm slide projector. It does not matter what you use but be sure to cut your specimens to fit the format of the slide before grinding.
All slides must be cleaned thoroughly. This is a most important step in making a successful slide. No slides are clean as delivered by the factory, in fact, they can't be. If the glass slides were perfectly clean cohesion of the glass surfaces would make it impossible for you to pry them apart. To avoid this manufacturers use a light dusting of powders like talc or thin films of silicone oils on the glass to keep them from sticking. This must be completely cleaned off for bonding to be strong enough to resist the tremendous shearing forces of polishing. The best method of cleaning is to use Tri-Sodium Phosphate (TSP) as a water soluble cleaner followed by denatured alcohol. I have successfully used Windex as the water soluble cleaner and 95% Isopropyl alcohol. You should check each slide for any smears or films in reflected light and take every effort to get the slides scrupulously clean.
Lay the slides on a layer of paper several sheets thick on a solid flat surface. Be sure the surfaces are clean. For bonding agents epoxy or any of the cyano-acrylic glues such as Super-Glue or PaleoBond can be used. I prefer the latter as they are a bit more brittle and thus grind better but they frequently require the application of a cover glass after polishing. Epoxies tend to be tougher but more plastic, which slows the grinding (actually the rock will grind much faster than the glue) and slows the polishing, plus the glue itself rarely polishes to clear. If you do use epoxy you should avoid the fastest setting glues (5 min) and go for the longer (20 min. or longer) as these have different chemical formulae and bond to the glass better.
With the cyano-acrylic glues you should individually put a drop or two (depending on the size of the piece) on the glass slide and take the specimen and push it into the drops, centering it on the slide and working it back and forth a bit so excess glue and bubbles are forced out the sides. If there is any dirt between specimen and glass it can break either of these or will form a wedge between them that will later ruin the slide. Unlike many other uses the glue will not bond with its usual quickness here. You have a minute or so to arrange the specimen. You may want to use popsicle stick or some such soft prod to push the specimen around as the glue will bond the slide or specimen to your fingers much faster than the specimen to the slide. It will take at least 24 and maybe 48 hours to get this glue to completely harden. After 24 hours it is advisable to go back and put a bead of the glue around the edge of the specimen to seal it in well. It also helps if the slide can be very mildly warmed. A screen over a lamp shade with slides resting on it in the warm flow of air works very well. They should not get hot, only warm.
If epoxy is used mix enough to do four or five slides at a time. Do not try to do too many at once. For your first attempt just do one or two. Take a Popsicle stick and wipe the glue on the specimen first to fill any pores. Then put a drop or two on the slide and push the sample into it working it back and forth a bit to seat it and get a good bead of glue around the outside. Do not use too much glue. It will become a messy job and a wide glue margin around the work will slow grinding and polishing. These will need 24 hours to bond completely before any grinding.
In both cases, after the glue has set but not bonded you can look at the backside to inspect for bubbles. It is unlikely that there will be any but wherever there are will be a weak spot in the slide that may cause problems later. There is virtually nothing that can be done to fix this so you will have to be aware of it as we go on and treat this slide with greater care.
Now that the little rock slab is glued to the glass microscope slide on it's polished side it is necessary to grind it down to the required thickness for transparency so you can literally see through rock without x-ray vision, even if the rock were kryptonite! The most important thing now is to make sure the front surface being ground becomes parallel to the glued down polished side, or the surface of the microscope slide itself. The latter is much easier.
For this task you will need some sort of shim stock. This is just thin metal that can be cut with scissors. It is desirable to have several thicknesses. Aluminum flashing will work though it is a bit soft. Better is brass shim stock available at hobby shops and hardware stores. Cut several strips from the thickest stock about 1/4" to 3/8" wide and the full length of at two times the width of the slide. Take one of the thickest strips and a blank microscope slide and wrap the piece of shim stock around the short dimension of the slide (see diagram at end of paper). This will be called the "clip" and two of these will be needed. One side of the clip will span the slide and the other will probably not or will overlap. Put the clips on the working slide so the side that spans the slide is on the rock or working side of the slide. There should be a clip on either side of the specimen. The object will be to grind the rock specimen down to the level of the clips. If you wrapped them carefully around the blank slide such that there is no bubble, bow or kinks in the clip surface, then when the specimen is ground down it will the same thickness across and parallel to the front surface of the slide.
A technique that has worked well is to grind the specimen down with 220 grit to the level of the first clip. This will be something less than 1mm thickness. Then wash up and go the next thickness clips and the next grit. By this time the specimen should be getting translucent when held up to a light and fossils should be visible though not clearly. With the second to the last grit (depending on how many grades you want to use, in my case the 5 micron grade) grind the slide down to almost the desired finished thickness. The last grade of abrasive will be used with no clips and should be used only to remove the pits from the previous grade and any final touch-up on the thickness or "wedge".
"Wedge" is when one side of the specimen is thicker than the other. As you hold it up to the light one corner or side will be darker than the other. The cause can be poor grinding, a defective clip, a bit of grit or dust under one end of the specimen when it was glued down or a combination of these. It can be corrected. When grinding in these final stages put the finger tip pressure over the darkest area. The slide and specimen are fairly flexible now and this will put a little more pressure on that spot causing it to grind a little more in that spot.
Go slow with the grinding in these stages, don't rush. You've put a lot of work into this and you don't want to wreck it now. Inspect the work frequently. When you can clearly see details in the fossils through the sample, go to the last grinding grit. If a fossil pops out of the sample or disintegrates in grinding, as can most often happen with fusulinids, or the specimen grinds through above a trapped bubble in the glue underneath, then stop and go to the final grind stage. If you are in the final grind when this happens then go straight to polish. This will not produce a high quality slide but it will save the slide to that point. To go on further with the grind would result in the disintegration of all or most of the slide. Better to have something than nothing!
Use the last grinding grit until the work has a shine to it when a light is reflected off of it at a low angle. Try to remove wedge but in a first effort this may not be successful. This is not a disaster. The last wet with the finest grinding grade should be with very light pressure and should just skid about on the glass tool. This will smooth out the pits of this final grade and make the polish go even easier.
Once you have a good grind where you can see a reflection of a light bulb off the work when viewed at a glancing angle (see diagram at end of paper), and the specimen is quite transparent, then it is time to polish. Again, the piece of denim or twill is wrapped around the well cleaned grinding tool and a charge of cerium is applied to the cloth. Polish until the adhesive is as clear as possible. The details in the fossils will become more and clearer as the specimen takes on a polish.
Occasionally the glue will begin to peel at this stage, especially the cyano-acrylic glues like PaleoBond. If this happens trim away some of the excess that is peeling with a razor blade or very sharp knife but don't overdo it. Leave the best margin around the specimen that you can. This peeling is a warning that the glue-glass bond is not as good as it could be. It will be necessary to put a cover glass over the specimen after the polishing.
If it is necessary to put a cover glass on then wash the specimen and slide well making sure all the polishing residue is gone. Then let it dry for a day. Obtain a set of microscope cover glasses or cover-slips from a hobby/science shop or from a scientific supply house. Also try and get a low viscosity cyano-acrylic glue and its solvent. The latter is most important. The larger cover glasses are best to ensure the entire sample is covered. I use some that are nearly 1x1.5 inches. If you are using the smaller ones (1x1cm) then one drop of thin cyano-acrylic glue is all that will be needed. In fact, it will probably be too much. Apply the drop and gently push the cover into it softly applying pressure. Watch out that the glue does not get on the fingers. Wipe the excess with tissue or paper towel and don't worry about smears at this stage. You will have about 30 seconds to place the cover glass correctly so waste no time. Your first effort may not be a work of art. We can fix some of that, but a crooked cover glass is permanent.
After a few minutes, when the cover glass is well affixed, get a tissue and put several drops of the solvent on it.
Use this to clean up the excess glue and smears. You'll be pleasantly surprised how readily these clean up. Stubborn drops and glops of glue can be coaxed free with a single-edged razor blade.
Other things that can be thin sectioned.
There is virtually no limit to the things you can section and enjoy. My own introduction to this was when the Arizona-Sonora Desert Museum (ASDM) asked me to look for fossils in the matrix from the Sonorasaurus site south of Tucson. As luck would have it, there was not a fossil to be found in the matrix with the possible exception of a tiny, fragmentary coprolite. The soil was very quartzitic and well sorted indicating a probable swift flowing stream deposition. But there were Sonorasaurus bone chips in the matrix and these did fascinate me. I then and there decided to make my first thin sections, and the only ones to date of the Sonorasaurus. When I saw the internal bone structure I was amazed. I had no idea such structure existed. Next, I did library research on bone histology and particularly books and papers on dinosaurian bone histology at the Univ. of Arizona Library and spent the summer of 1996 pouring over them and making thin sections of all the dino-bone I could lay my hands on. I had an idea that perhaps by measuring the diameters of the cellular structures in the dense bone, called Haversian Systems, I could make comparisons with other dinosaurs. My results are summarized in the paper given at the Fossil in Arizona conference for that year and co-authored by Ron Ratkevich formerly with the ASDM. Statistically, the secondary osteon diameters most resembled hadrosaurians and sauropods and not ceratopsians or carnosauria. (Reid, R.E.H. 1966)
I then embarked on a thin section exploration of many fossils and fossiliferous rocks. I have made sections of bones from the Devonian through the Pliocene and teeth as well. This technique lends itself well to making good use of bone chips and fragments, particularly if the parent creature is known. My wife, a meteoriticist with Lunar & Planetary Lab at Univ. of Arizona, has pressed my talents into service on igneous rocks as well, but they're not nearly as interesting now are they?
So the only limit to what you can section is your imagination. It is a powerful tool in determining ages, populations and doing statistics on all sorts of fossil facies.
How to stabilize crumbly matrix or bones.
One problem that does come up with some materials is stability of the fossils and/or matrix. This can be dealt with by several methods. For the most porous materials, soak the little sample with cyano-acrylic solvent and immediately put glue on it so as the solvent evaporates it pulls the glue into the specimen. Let this dry for a day or so and then put more of the glue on the outside. It should be hard enough by then to allow you to proceed with the processing.
If a specimen, particularly hard brittle bone, crumbles from the perimeter as you grind, it may be best to coat it with epoxy or even encase it in a block of the material. This is frequently the case when working teeth. Broken and chipped teeth are cheap and very identifiable as to the original owner, usually right down to the species. This makes them very desirable as specimens. But they are also crumbly because of the strong layering. So it is usually necessary to encase them in epoxy just to keep them in one piece even after soaking them as above.
One word of warning from the opposite side of this problem. Some things are incredibly hard and resistant to grinding. Notably among these are dinosaur egg shells. The shell of the Titanosaur from Argentina is the hardest thing I have ever had to grind and just to get it down to a millimeter it was necessary to use a grinding wheel! The smell is strong and oily, worse than any Green River shale.
Fixing broken slides.
It will happen that you will break a slide. As Murphy would have it, this will be one of your better works. All is not lost as long as the break does not occur through the specimen, but the remedy is tricky. Take another microscope slide the same size as the broken one and using the mounting glue (either type) glue the broken one to the new slide aligning it as best you can. Epoxy may be preferable for this job since it will not set up as fast as the cyano-acrylic glue but it is more prone to bubbles on such a large surface. Put a large drop or two in the middle of the slide and push the other squarely into the work. Work the two slides against each other well to force out any bubbles and don't use too much glue or the job will get quite messy.
This is done by running the slide up and down along the straight side with the portion to be frosted grinding on the surface of the tool. Finding a permanent but still small enough font labels was difficult and is still an ongoing process. The best solution so far has been the old tried and true method of hand lettering with pen, nib and India ink on a frosted end of the slide. We know this will work and last for at least a century. If your slides do not already have a frosted or fine ground end on them it will be necessary to make one. This is simply done by using a flat piece of glass with one straight side. , some 400 or 600 abrasive and grinding the section you want (see diagram at end of paper).
Keep good records. This is very important should you find something new and unusual. Unlike with megafossils as there is an additional stage of separation from the original source. With an ordinary megafossil the specimen is collected in the field catalogued, stored and labeled. With microfossils, particularly with thin sections, the specimen is collected, cut out of the original matrix, prepared, then catalogued, stored and labeled. So it is very important that it be traceable back to the point of origin as specifically as possible. This does not mean that an unidentified fossiliferous rock cannot be useful. Thin sections from such a rock can be educational and just plain fun to look at.
Frequently, when at a show, a particularly interesting fossil laden rock will catch our eye. It will be poorly labeled or unlabelled but is reminiscent of several possible formations. A thin section, followed by analysis of the biota therein can, in many cases, nail the point of origin!
You do not need a fancy Bausch & Lomb binocular microscope with camera attachments and special illuminators to enjoy your slides. There are a number of much more economical means to view your work. Radio Shack and some hobby shops have very economical hand held microscopes at reasonable prices. These are alright for a single viewer but what about a bunch of people. As I said before, I make some of my slides in 35mm slide size. The specimen is usually made to fit the opening in a 35mm slide mount so when I am finished I can glue a 35mm slide frame to one side of the glass, to prevent breakage and to block light around the specimen. Then I only need to project the prepared slide like any other photographic slide and I have a microscope with which I can study my slide in comfort without having to squint through an eyepiece.
Another relatively inexpensive, low powered microscope is the old microfiche reader. These can be had at junk stores, garage sales and going-out-of-business sales for very little money. The slide, in fact up to half a dozen, can be put in the carrier at one time and studied by many people or yourself in relaxed comfort again not having to squint through the eyepiece.
What to do with these?
Now you may proudly examine your handiwork. There may be a couple bubbles in it and it may be a bit thicker on one end than the other, but still it will show you a whole new world of fossil flora and fauna that will fascinate you for hours. Try making sketches of the things you see and then identify them. A lot will be learned from this process. Build a library of papers and books on the formations you wish to study. Identify the fossils in your sections.
It is a good idea to get a book on paleontological thin sections. These have been published by oil companies, societies and associations of economic geologists and paleontologists and are not too costly on the used market. (Johnson)
Learn to make sketches of these fossils and keep a book of them. Many will not be identifiable at first. Keep at it. It may take years to make positive identifications of all the fossils on a given slide during which time you will have undoubtedly made more slides that will also have unidentified creatures on them. Eventually you may even discover a new one. This is very possible in the world of microfossils and thin sections.
References: Allman, Michael, and Lawrence, David F., (1972) Geological Laboratory Techniques, ARCO Pub. Co., Inc, New York. Johnson, J.H., (no year) Recognition of Organic Debirs in Thin Sections of Limestone, Tenneco Oil Co. Jones, Daniel J., (1956) Introduction to Microfossils, Harper & Brothers, p.7-18. Kummel, B., and Raup, D., (1965) Handbook of Paleontological Techniques, W.H.Freeman and Co., San Francisco. Reid, R.E.H., (1996) "Bone Histology of the Cleveland-Lloyd Dinosaurs and of Dinosaurs in General, Part I: Introduction: Introduction to Bone Tissues", Brigham Young Univ. Geological Studies, Vol.41.